We plan to continue studies with temperature-sensitive mutants of simian papovavirus SV40 in order to determine the cistron responsible for the enhancement of replication of human adenoviruses in simian cells. We have also shown that the ability of simian cells to support the replication of human adenoviruses in the absence of SV40 can be enhanced by pretreating the cells with 5'-iodo-deoxyuridine (IUdR). A somewhat similar system appears to result from the pretreatment of human cells with IUdR prior to infection with SV40. We will attempt to determine the mechanisms by which pretreatment with DNA analogs can change the susceptibility of a cell to virus infection. Other studies involve the ability of viruses photodynamically inactivated following treatment with neutral red to induce cell transformation. The properties of such inactivated viruses will be characterized and the limits of genome expression in the transformed cells determined. An in vitro model system for the study of latent measles virus infections has been developed and will be characterized.